8.5.1.1.1.1 UART Transmitter

The transmitter is configurable to wait a specified number of bit periods after the last bit reception before starting transmission using the TX_HOLD_OFF register. This provides time for the host to switch the bus direction at the end of its transmission. The TX hold off time for base and stack can be calculated as below:

1. Generic formulas to calculate the actual TX hold (bit period) of time for STACK devices:
   1. Minimum=TX_HOLD_OFF x Bit Period
   2. Typical= 22.5+7x(Number of Devices-2)+(TX_HOLD_OFF+1.5) x Bit Period
   3. Maximum= 24.5+9x(Number of Devices-2)+(TX_HOLD_OFF+4.5) x Bit Period
2. Generic formulas to calculate the actual TX hold (bit period) of time for BASE devices:
   1. Minimum=TX_HOLD_OFF x Bit Period
   2. Typical= 1.5+(TX_HOLD_OFF+1.5) x Bit Period
   3. Maximum= 2.5+(TX_HOLD_OFF+2.5) x Bit Period

Note that the total number of devices includes all stack devices and the base. Also note that the Bit Period depends on the baud rate.

When the device receives a communications reset, the baud rate for the UART is reset to 250kbps. The baud rate is programmable by the host to a higher or lower rate by writing to COMM_CTRL[BAUD]. The UART TX is disabled/enabled using the COMM_CTRL[UARTTX_EN]. Once disabled, no responses are transmitted. The transmitter is disabled immediately following the disable command.